Method of waterproofing piezoelectric crystals and waterproofed crystal unit



May 28, 1957 P. P. ZAPPONI 2,794,132

METHOD OF WATERPROOFING PIEZOELECTRIC CRYSTALS AND WATERPROOFED CRYSTAL UNIT Filed Sept. 23, 1953 FIG. 3

INVENTOR. PASCHAL P. ZAPPONI TEMPERATUR E IN DEGREES CENTIGRADE ATTO N EY METHOD OF WATERPROOFING PIEZOELECTRIC CRYSTALS AND WATERPROOFED CRYSTAL UNIT Paschal P. Zapponi, Cleveland, Ohio, assignor to (Ilevite Corporation, Cleveland, Ohio, a corporation of Ohio Appiication September 23, 1953, Serial No. 381,896

6 Claims. (Cl. 310-33) This invention relates to a method of making waterproofed assemblies of piezoelectric crystals containing water of crystallization, particularly Rochelle salt crystals, and to a waterproofed crystal assembly of this type.

As is well understood, electroded Rochelle salt piezoelectric crystal units are vulnerable to hot, humid atmospheres due to the absorption of water from the atmosphere, which causes the resistivity of the units to fall below usable values. Among the various methods attempted for waterproofing these units to increase their operation life under hot, humid atmospheric conditions, excellent results have been obtained by a method which involves wrapping the unit in an envelope of aluminum foil approximately .001 inch thick covered on one or both sides by a moisture resistant, plastic film of substantially the same thickness. This protective wrapper is open at the end of the crystal from which the electrical leads project and at the end opposite therefrom. To complete the moisture seal afforded by this wrapper, it has been the practice to fill these open ends of the wrapper with molten micro-crystalline wax which has water vapor barrier properties substantially as good as the metal wrapper itself. However, it has been found necessary to perform this wax depositing operation by hand and with extreme care, which has increased substantially the cost of assemblies moisture proofed by this method. It is not feasible to dip the open ends of the foil wrapped crystals in molten Wax (at 150 C.) because the crystal itself melts at the much lower temperature of 50 C. In addition, if any wax should overflow the ends of the foil wrapper this makes it difficult to attach the usual moun ing pads for the crystal employed in hearing aids, phonograph pickup cartridges, and the like, because of the difficulty of adhering to wax.

These disadvantages attendant to the foregoing prior art practices are avoided in the present invention by a process which requires less precision on the part of the operator and which therefore results in a substantial improvement in economy.

Accordingly, it is an object of the present invention to provide a novel and more economical method of making a moisture proofed crystal assembly having a piezoelectric crystal unit of the type which contains water of crystallization.

Another object of this invention is to provide a novel method of making moisture proofed Rochelle salt crystal assemblies in which the piezoelectric crystal units are so treated that they are capable of absorbing increased quantities of moisture before becoming defective, without in any way interfering with the piezoelectric action of the crystal units.

Also, it is an object of the present invention to provide a novel, economical, moisture proofed Rochelle salt crystal assembly capable of satisfactory operation under humid conditions for an extended period of time.

The foregoing objects are preferably accomplished in the present invention by wrapping the electroded crystal unit in coated metal foil, as before, and then dehydrating nited States Patent 2,794,132 Patented May 28, 1957 the unwrapped ends of the crystal unit and dipping these ends in a lacquer, which after drying forms a moisture resistant resinous film thereat substantially less effective as a vapor barrier than that provided by the wrapper itself. As is conventional in crystal units of this type, these ends of the crystal unit are not contacted by the electrodes, and hence are not effective in the piezoelectric action of the crystal, so that this dehydration of the ends is in no way deleterious to the piezoelectric performance of the crystal units. Because of their less effective vapor seal, these lacquer sealed ends of the waterproofed crystal assembly are most vulnerable to the diffusion of water vapor from the atmosphere into the crystal. However, a very substantial amount of this water vapor must be absorbed by the anhydrous ends of the crystal unit in returning to the normal hydrated state before the water vapor can cause any deteriorating supersaturation of the electroded middle portion of the crystal unit which determines its performance.

Other and further objects and advantages of the present invention will be apparent from the following description, with reference to the accompanying drawing.

In the drawing:

Figure 1 is a perspective view of the crystal unit and the laminated foil wrapper therefor prior to wrapping this wrapper around the crystal;

Figure 2 is a perspective view showing the crystal unit wrapped in the foil wrapper, prior to the sealing of the open ends of the wrapper;

Figure 3 is a transverse section through the wrapped crystal unit of Figure 2; and

Figure 4 is a solid-vapor state phase diagram for Rochelle salt.

Referring to Figure 1, the piezoelectric crystal unit is shown for illustrative purposes as being in the form of two rectangular Rochelle salt crystal plates 10 and 11 cemented together at confronting major faces. The crystal plates may have their respective crystallographic axes arranged in any desired manner, for example, as specified in U. S. Letters Patent to Sawyer, Reissue No. 20,213 or Reissue No. 20,680, depending upon the piezoelectric action desired. An electrode may or may not be secured between the confronting faces of the crystal plates, depending upon whether a series or parallel connection of these crystal plates is desired.

Each exposed outer major face of the crystal unit is coated with a graphite electrode 13, which covers the major portion of the crystal face at the middle thereof, leaving non-electroded marginal segments 14, 15, 16 and 17 at the respective edges of the crystal unit. As is well understood, these non-electroded marginal portions of the crystal unit have no effect on its piezoelectric action. Normally, the weight of the non-electroded marginal portions 1417 will total about 10% of the weight of the crystal unit.

Electrical leads 18 and 19 are secured to the electrodes 13 by means of metal foil strips 20 cemented to both. The leads 18 and 19 project beyond the end of the crystal unit at which the marginal end portion 16 is located.

As a moisture resistant envelope or enclosure for substantially all of the crystal unit there is provided a thin flexible sheet 21 consisting of aluminum foil about .001 inch thick coated on both sides, or on the side touching the graphite electrode only, with a suitable moisture resistantplastic film of the same thickness. This coated foil wrapper is the same as those previously used for the same purpose in waterproofed crystal assemblies of different types. A tacky adhesive is provided on the inner face of this sheet for securing it to the crystal unit. As shown in Figure 1, the width of foil sheet 21 exceeds that of the crystal unit, so that the foil sheet projects beyond each end of the crystal unit about 4 to of an inch. The foil sheet is wrapped tightly around the crystal with its ends overlapping, as shown in Figures 2 and 3. Obviously, this leaves the ends 14 and 16 of the crystal unit unprotected at unwrapped opposite ends of the assembly. I

In accordance with the present invention, the marginal portions Hand 16 at the unwrapped ends of the unit are first dehydrated before moisture resistant material is applied to fill the open ends of wrapper 21. The amount of water of crystallization to be removed from the crystal unit may be computed readily, knowing the combined weight of the crystal plates 11 and 12 and the percentage of these crystal plates occupied by their non-electroded marginal end portions 14 and 16. Knowing the' Weight of the crystal unit, the weight of the total water of crystallization therein may be determined, and from this the weight of water in the non-electroded marginal end portions 14 and 16 may be determined by multiplying the total weight of water in the crystal unit by the percentage of the crystal unit contained in the non-electroded marginal end portions 14 and 16. Usually, the non-electroded marginal end portions 14- and 16 represent at least 5% of the total crystal, so that in such event it would be safe to remove 5% of the Water from the crystal to dehydrate these marginal end portions without danger of dehydrating any of the electroded portion of the crystal and thus changing the piezoelectric action of the crystal unit.

The dehydration of the unwrapped ends of the crystal may be efiected by exposing it for a period of time to a desiccant, such as silica gel, aluminum oxide, anhydrous calcium sulfate or calcium chloride, or by heating the crystal, or by exposing the crystal to a vacuum, or by any combination of desiccants, heat and evacuation. Obviously, heating the crystal at the same time as exposing it to a desiccant will cause the water of crystallization to be removed much more rapidly than exposing it to a desiccant without heating it.

After the desired amount of water has been removed from the crystal unit to dehydrate the non-electroded ends 14 and 16, without dehydrating the electroded middle portion of the crystal which determines its piezoelectric action, the unwrapped ends of the Figure 2 assembly are dipped in a moisture-resistant lacquer at room temperature to fill the open ends of wrapper 21 and seal the ends 14 and 16 of the crystal. In one practical embodiment, the crystals were thus dipped four times in a clear, parlon lacquer and then dried for 16 hours at 4045 C. In drying at an elevated temperature in this manner the solvents in the lacquer are evaporated, r

leaving a resinous moisture-resistant film sealing the unwrapped ends of the crystal unit.

Since the lacquer film at the ends of the assembly is substantially less effective as a moisture barrier than the foil wrapper, difiusion of water vapor into the crystal will occur first through the lacquer seal into the crystal. The advantage of having the ends of the crystal which are protected only by this lacquer seal anhydrous will be apparent from a consideration of the solid-vapor state phase diagram for Rochelle salt shown in Figure 4. The lower curve in this figure represents the equilibrium condition between hydrated Rochelle salt (NaKC4H4OG 4H20) and anhydrous Rochelle salt. Thus, assuming a relatively severe operating temperature of 40 C., the point E on this lower curve represents the equilibrium condition for Rochelle salt at which the crystal has a water vapor pressure represented by the value 52% relative humidity. This is the condition of the crystal when it is partly hydrated and partly anhydrous. If the crystal were completely anhydrous it would have a relative humidity of 0%. If the crystal were completely hydrated, because of the permeability of its waterproofing it would ultimately assume the relative humidity of the surrounding thefully hydrated state.

4' atmosphere, if the latter is higher than 52%. At atmospheric humidities below 52%, a completely hydrated crystal would lose water and become partly anhydrous, but in no event would its own vapor pressure drop below 52% relative humidity while the crystal is at least partly hydrated.

In the present instance, having dehydrated the ends of the crystal, the crystal is partly hydrated and partly anhydrous, so that the equilibrium condition at point B on the lower curve in Figure 4 would prevail. When exposed to surroundings of higher humidity, the anhydrous ends of the crystal will gradually absorb water from the surrounding atmosphere until the crystal is finally fully hydrated. However, during this initial period when it is becoming increasingly hydrated, the vapor pressure of the crystal will remain at 52% relative humidity.

Thereafter, assuming that the protective material for the crystal is permeable to water vapor, the completely hydrated crystal eventually will assume the same vapor pressure as that of the surrounding atmosphere. Should the relative humidity of the atmosphere be about 86% or above at 40 C., the hydrated Rochelle salt crystal would dissolve, as indicated by the upper curve in Figure 4, representing the equilibrium state betweenthe hydrated crystal and the saturated solution of the crystal in contact with the hydrated salt. Actually, for practical purposes the hydrated Rochelle salt crystal unit becomes useless When its vapor pressure reaches about 79% relative humidity at 40 C. because at this point the resistivity of the unit drops below a usable value.

The present invention takes advantage of the con siderable time period required for the partly anhydrous crystal to absorb suflicient moisture to be converted to Because the moisture resistant coating (lacquer) for the anhydrous ends of the crystal is less efiective as a moisture barrier than the foil wrapper for the rest of crystal, these ends are most susceptible to the difiusion of water vapor into the crystal. However,

before the vapor pressure of the crystal can rise above 52% at 40 C., the anhydrous ends of the crystal must first absorb sufiicient moisture until they are fully hydrated. After the entire crystal has become hydrated it continues to absorb moisture from its high humidity surrounding until the vapor pressure of the electroded portion of the crystal rises to a point where the resistivity of the crystal unit is so low as to render it useless.

With the foregoing arrangement it has been found that the crystals have acceptable life under high temperature, high humidity conditions. The particular arrangement in which the less eifectively protected ends of the crystal are anhydrous results in a very considerable time period for these ends of the crystal to become hydrated, and during this time the vapor pressure of the crystal cannot rise above 52%. Thus, with this novel arrangement the life of the crystal is prolonged, while at the same time it is practical to use a relatively inexpensive lacquer dipping operation to seal the unwrapped ends of the crystal and there is no necessity to resort to careful hand operations which are required when wax is used as the sealing material.

While in the foregoing description there is disclosed a particular preferred embodiment of the present invention, it is to be understood that various modifications,

omissions and refinements departing from the disclosed embodiment may be adopted without departing from the spirit and scope of the invention.

Iclaim:

1. A method of making a waterproofed crystal assembly which comprises the steps of: providing a water soluble crystal unit containing water of crystallization; applying electrodes to portions only of the major faces of the crystal unit, leaving a non-electroded marginal portion of the crystal unit which is ineffective in the piezoelectri action of the crystal unit; wrapping moisture rep sistant material across the electroded portions of the crystal unit, leaving said non-electroded marginal portion of the crystal unit exposed; dehydrating said exposed nonelectroded portion of the crystal unit to render said nonelectroded portion anhydrous without dehydrating the crystal material between the electrodes so that substantial water vapor may be absorbed by said anhydrous portion of the crystal without atfecting the piezoelectric action of the crystal unit before the crystal is fully hydrated and thus capable of increasing in vapor pressure; and after said dehydrating step sealing the exposed non-electroded marginal portion of the crystal unit with moisture resistant material.

2. A method of making a moisture proofed piezoelectric crystal assembly which comprises the steps of: providing a water solvent piezoelectric crystal unit containing water of crystallization; applying electrodes to portions only of the major faces of the crystal unit, leaving non-electroded marginal portions on said major faces at two opposite ends of the crystal unit, at least, which are ineffective in the piezoelectric action of the crystal unit; enclosing in a moisture resistant wrapper all of the crystal unit except said two opposite ends thereof; dehydrating said unwrapped ends of the crystal unit to render the crystal unit anhydrous thereat without dehydrating the crystal material between the electrodes; and, following said dehydrating step, sealing the unwrapped anhydrous ends of the crystal unit with moisture resistant liqaid at a temperature below the melting point of the crystal.

3. A method of making a waterproofed Rochelle salt crystal assembly which comprises the steps of providing a Rochelle salt crystal unit; applying electrodes to the crystal unit contacting portions only of the major faces of the crystal unit, leaving non-electroded marginal portions on said major faces at two opposite ends of the crystal unit, at least, which are ineffective in the piezoelectric action of the crystal unit; enclosing in a moisture resistant wrapper all of the crystal unit except said two opposite ends thereof, with said wrapper extending slightly beyond said opposite ends of the crystal unit; dehydrating said unwrapped ends of the crystal unit to render the crystal material anhydrous thereat Without dehydrating the crystal material between the electrodes; following said dehydrating step sealing the unwrapped anhydrous ends of the crystal unit with moisture resistant liquid at a temperature below the melting point of the crystal material; and drying said liquid to leave a moisture resistant film sealing the anhydrous ends of the crystal unit.

4. A method of making a waterproofed Rochelle salt crystal assembly which comprises the steps of: providing a Rochelle salt crystal unit; applying electrodes to portions only of the major faces of the crystal unit, leaving nonelectroded marginal portions on said major faces at two opposite ends of the crystal unit, at least, which are ineffective in the piezoelectric action of the crystal unit; wrapping a flexible sheet of moisture resistant metal foil having a width slightly greater than the spacing between said opposite ends of the crystal unit around all of the crystal unit except said two opposite ends thereof, with the metal foil wrapper projecting slightly beyond each of said opposite ends of the crystal unit; dehydrating said unwrapped opposite ends of the crystal unit to render the crystal material anhydrous thereat without dehydrating the crystal material between the electrodes; and, following said dehydrating step, sealing between the metal foil wrapper and the unwrapped anhydrous ends of the crystal unit moisture resistant lacquer.

5. A method of making a waterproofed Rochelle salt crystal assembly which comprises the steps of: providing a Rochelle salt crystal unit; applying electrodes extending across middle portions only of the major faces of said crystal, leaving non-electroded marginal portions on said major faces at two opposite ends of the crystal unit, at least, which are ineffective in the piezoelectric action of the crystal unit, and having electrical leads connected to said electrodes extending beyond one of said ends of the crystal unit; wrapping a flexible sheet of metal foil having a width slightly greater than the spacing between said opposite ends of the crystal unit around the electroded crystal unit to completely enclose the electroded major faces of the crystal unit and the edges of the crystal unit between said opposite ends thereof and leaving the wrappedaround sheet open at said opposite ends of the crystal unit and projecting slightly beyond said ends of the crystal unit; dehydrating said marginal portions of the crystal unit at said opposite ends thereof to render said marginal portions anhydrous while at the same time maintaining the crystal material between the electrodes hydrated; following said dehydrating step dipping said unwrapped anhydrous ends of the crystal unit in lacquer; and evaporat ing the solvents from the lacquer to leave a moisture resistant resinous film across said anhydrous ends of the crystal unit.

6. A moisture proofed crystal assembly, comprising a Rochelle salt crystal unit, electrodes contacting portions only of the major faces of the crystal unit and leaving non-electroded marginal portions on said major faces at two opposite ends of the crystal unit, at least, which are inefiective in the piezoelectric action of the crystal unit, a moisture resistant metal foil wrapper closely surrounding the crystal unit entirely therearound except at said opposite ends thereof, said wrapper being open at said opposite ends of the crystal unit and extending slightly beyond the ends of the crystal unit thereat, said marginal portions of the crystal unit at said opposite ends thereof being anhydrous, and a continuous film of dry moisture resistant lacquer filling said open ends of the wrapper to seal said anhydrous ends of the crystal unit.

References Cited in the file of this patent UNITED STATES PATENTS 2,433,383 Mason Dec. 30, 1947 2,483,677 Swinehart Oct. 4, 1949 2,493,145 Jaffe Jan. 3, 1950 

1. A METHOD OF MAKING A WATERPROOFED CRYSTAL ASSEMBLY WHICH COMPRISES THE STEPS OF: PROVIDING A WATER SOLUBLE CRYSTAL UNIT CONTAINING WATER OF CRYSTALLIZATION; APPLYING ELECTRODES TO PORTIONS ONLY OF THE MAJOR FACES OF THE CRYSTAL UNIT, LEAVING A NON-ELECTRODED MARGINAL PORTION OF THE CRYSTAL UNIT; WHICH IS INEFFECTIVE IN THE PIEZOELECTRIC ACTION OF THE CRYSTAL UNIT; WRAPPING MOISTURE RESISTANT MATERIAL ACROSS THE ELECTRODE PORTIONS OF THE CRYSTAL UNIT, LEAVING SAID NON-ELECTRODED MARGINAL PORTION OF THE CRYSTAL UNIT EXPOSED; DEHYDRATING SAID EXPOSED NONELECTRODED PORTION OF THE CRYSTAL UNIT TO RENDER SAID NONELECTRODED PORTION ANHYDROUS WITHOU DEHYDRATING THE CRYSTAL MATERIAL BETWEEN THE ELECTRODES SO THAT SUBSTANTIAL WATER VAPOR MAY BE ABSORBED BY SAID ANHYDROUS PORTION OF THE CRYSTAL WITHOUT AFFECTING THE PIEZOELECTRIC ACTION OF THE CRYSTAL UNIT BEFORE THE CRYSTAL IS FULLY HYDRATED AND THUS CAPABLE OF INCREASING IN VAPOR PRESSURE; AND AFTER SAID DEHYDRATED STEP SEALING THE EXPOSED NON-ELECTRODED MARGINAL PORTION OF THE CRYSTAL UNIT WITH MOISTURE RESISTANT MATERIAL. 